course phy121 Jؽv{Ñassignment #012
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21:30:46 Query set 3 #'s 13-14 If an object of mass m1 rests on a frictionless tabletop and a mass m2 hangs over a good pulley by a string attached to the first object, then what forces act on the two-mass system and what is the net force on the system? What would be the acceleration of the system? How much would gravitational PE change if the hanging mass descended a distance `dy?
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RESPONSE --> The forces that act on the system are the forces created by gravity acting on the hanging object. (g * m1)/(m1+m2)=a `dPE = - g m1 `dy confidence assessment:
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21:31:38 ** The net force on the system is the force of gravity on the suspended weight: Fnet = m2*9.8m/s/s Gravity also acts on m1 which is balanced by force of table on m1, so this force makes no contribution to Fnet. Acceleration=net force/total mass = 9.8 m/s^2 * m2 / (m1+m2). If the mass m2 descends distance `dy then gravitational PE decreases by - m2 g * `dy. COMMON MISCONCEPTIONS AND INSTRUCTOR COMMENTS: The forces acting on the system are the forces which keep the mass on the table, the tension in the string joining the two masses, and the weight of the suspended mass. The net force should be the suspended mass * accel due to gravity + Tension. INSTRUCTOR COMMENT: String tension shouldn't be counted among the forces contributing to the net force on the system. The string tension is internal to the two-mass system. It doesn't act on the system but within the system. Net force is therefore suspended mass * accel due to gravity only 'The forces which keep the mass on the table' is too vague and probably not appropriate in any case. Gravity pulls down, slightly bending the table, which response with an elastic force that exactly balances the gravitational force. **
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RESPONSE --> I think I confused m1 and m2 as to which was on the table and which was suspended now that I think about it. self critique assessment:
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21:32:06 How would friction change your answers to the preceding question?
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RESPONSE --> Friction would act to reduce the Fnet acting on the system. confidence assessment:
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21:32:18 **Friction would act to oppose the motion of the mass m1 as it slides across the table, so the net force would be m2 * g - frictional resistance. **
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RESPONSE --> self critique assessment:
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21:33:38 Explain how you use a graph of force vs. stretch for a rubber band to determine the elastic potential energy stored at a given stretch.
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RESPONSE --> It would allow you to follow the curve to the point at which your rubber band is stretched. At this point of x, you could follow it to the y axis to determine how much PE the rubber band has at this point. confidence assessment:
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21:34:57 ** If we ignore thermal effects, which you should note are in fact significant with rubber bands and cannot in practice be ignored if we want very accurate results, PE is the work required to stretch the rubber band. This work is the sum of all F * `ds contributions from small increments `ds from the initial to the final position. These contributions are represented by the areas of narrow trapezoids on a graph of F vs. stretch. As the trapezoids get thinner and thinner, the total area of these trapezoids approaches, the area under the curve between the two stretches. So the PE stored is the area under the graph of force vs. stretch. **
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RESPONSE --> I thought that the y-axis would provide the measurement for stored PE, I see now that this was incorrect. PE would be the area under the graph, and the y-axis represents the force applied to the rubberband/force the rubber band is applying. self critique assessment:
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21:35:20 STUDENT QUESTIONS: Does the slope of the F vs stretch graph represent something? Does the area under the curve represent the work done? If so, is it work done BY or work done ON the rbber bands?
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RESPONSE --> confidence assessment:
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21:35:34 ** Slope isn't directly related to any physical quantity. The area is indeed with work done (work is integral of force with respect to displacement). If the rubber band pulls against an object as is returns to equilibrium then the force it exerts is in the direction of motion and it therefore does positive work on the object as the object does negative work on it. If an object stretches the rubber band then it exerts a force on the rubber band in the direction of the rubber band's displacement, and the object does positive work on the rubber band, while the rubber band does negative work on it. **
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RESPONSE --> self critique assessment:
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21:37:36 Query Add comments on any surprises or insights you experienced as a result of this assignment.
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RESPONSE --> My question here is actually about tests. Are we allowed to use a formula sheet? I hate to be ""that guy"" but I havn't memorized a formula since high school because I keep my books and write all formulas in the back of them so I don't have to worry about remembering them. I can just refer to them as I need them, which is nice since my memory isn't anything to brag about. Thanks for the info. self critique assessment:
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